1. Field of the Invention
The present invention relates to an image sensing apparatus being suitable for utilizing in a camera, which can take a motion picture and a still picture.
2. Description of the Related Art
There existed a solid state image sensing device as an image sensing apparatus being generally utilized in a video camera or like. FIG. 7 is a plan view of a solid state image sensing device partially enlarged. In FIG. 7, a solid state image sensing device is composed of a plurality of photoelectric converting elements 1, which converts an incident light into an electric signal and outputs, a vertical transmitting CCD (hereinafter referred to VCCD) 2, which transmits an electric charge outputted from the photoelectric converting element 1 in a vertical direction, and a horizontal transmitting CCD (hereinafter referred to HCCD) 3, which transmits the electric charge transmitted from the VCCD 2 in a horizontal direction. An electric charge transmitting method of a solid state image sensing device adopting a so-called progressive scan system, which can output a signal for one frame during a period of one field (hereinafter referred to 1 VD period) of television (TV) signal is explained briefly.
The photoelectric converting element 1 is disposed by, for example, 960 pixels in the horizontal direction and 640 pixels in the vertical direction. A reading out pulse is generated once a period of 1 VD of TV signal by a timing generator provided externally. All electric charges stored in each photoelectric converting element 1 are transmitted to the neighboring VCCD 2.
The VCCD 2 is stacked as a stage as many as a number of the photoelectric converting elements 1 disposed in the vertical direction. A stored electric charge is shifted in the HCCD 3 direction shown by an arrow V in FIG. 7 by one stage for a half period of one horizontal scanning period (hereinafter referred to 1 HD) of TV signal. In other words, an electric charge is shifted by two stages per 1HD period in order to be able to transmit electric charges equivalent to two lines per 1 HD period, that is, (960×2)=1920 pixels.
Further, the HCCD 3 is arranged as a stage as many as a number of the photoelectric converting elements 1 disposed in the horizontal direction. An electric charge from the VCCD 2 is transmitted in the horizontal direction shown by an arrow H in FIG. 7 and outputted from an outputting section 3 out to a picture signal processing section (not shown). A transmission rate of electric charge in the horizontal direction is set to a rate of transmitting electric charges equivalent to two lines per 1 HD period, that is, (960 pixels×2)=1920 pixels.
As mentioned above, in a case of utilizing a solid state image sensing device arranged by photoelectric converting elements 1 disposed by 960 pixels in the horizontal direction and 640 pixels in the vertical direction, a signal equivalent to 640 lines can be outputted if signals of all lines in the vertical direction are outputted. However, in a case of a TV signal of the NTSC system, for example, a number of horizontal scanning lines is 525 lines. Therefore, signals of all lines are not necessary to be outputted from a solid state image sensing device.
That is to say, in a case of outputting a signal as a TV signal of the NTSC system, it is sufficient to output a signal equivalent to 525 lines allocated in a vicinity of center of a picture composed of 640 lines vertically. Further, in a case of assuming that a signal equivalent to 2 lines per 1 HD period is outputted, since 1 HD period is equal to 63.5 μs, an electric charge can be horizontally transmitted by a clock of (960 pixels ×2÷63.5 μs)=30.24 MHz.
In addition thereto, another photoelectric converting element (not shown) for outputting an optical black (OB) signal is generally provided on the right and left sides of a solid state image sensing device in order to set a black level of a signal to be outputted by a photoelectric converting element. The OB signal is also transmitted through the VCCD 2 and the HCCD 3, so that a clock actually utilized is a little higher frequency than 30.24 MHz.
The above-mentioned description is one example of outputting a signal obtained from a solid state image sensing device as a motion picture signal such as a TV signal. However, there existed a video camera having a function of electronic still camera. Such a video camera outputs a TV signal by using a signal equivalent to 525 lines, for example, when outputting a motion picture. On the other hand, when outputting a still picture, one still picture can be obtained by outputting signals of all lines.
Recently, a still picture taken by a video camera is demanded for higher resolution. In order to obtain a still picture in high resolution, it is necessary for a solid state image sensing device to increase a number of pixels.
In a case of utilizing a solid state image sensing device composed of the photoelectric converting elements 1 disposed by, for example, 960 pixels in the horizontal direction and 640 pixels in the vertical direction, a still picture equivalent to approximately six hundred thousand pixels can be obtained. However, in a case of a still picture equivalent to approximately six hundred thousand pixels, printing the still picture with enlarged by using a home use printer can not obtain a picture in sufficient picture quality as a particle of pixel is extremely conspicuous.
Further, there existed various signal systems other than the NTSC system. In a case that a number of pixels of a solid state image sensing device is increased in the horizontal and vertical directions, a motion picture in other signal system than the NTSC system can be outputted. For example, in a case of considering a solid state image sensing device composed of the photoelectric converting elements 1 disposed by 1280 pixels in the horizontal direction and 720 pixels in the vertical direction, a motion picture by a system of progressively scanning 720 effective scanning lines by a frame frequency of 30 Hz, that is, the 720p/30 frame system can be outputted. However, in order to output such a motion picture, it is necessary for a solid state image sensing device to increase a number of pixels in the horizontal and vertical directions.
For the reason mentioned above, a solid state image sensing device having a large number of pixels is required. However, in a case that a number of pixels is increased, it is necessary for pixels to be horizontally transmitted furthermore during 1 HD period, so that a transmission rate of electric charge in the HCCD 3 must be increased. For example, with assuming that a number of pixels to be horizontally transmitted during 1 HD period is (1280×2)=2560 pixels, since 1 HD period is equal to 63.5 μs, a horizontal transmission rate becomes (1280 pixels×2÷63.5 μs)=40.34 MHz. However, if a transmission rate of electric charge in the HCCD 3 is increased such as mentioned above, there is existed a problem such that power consumption of the HCCD 3 increases. Further, a signal to noise ratio is deteriorated by heat generation of the solid state image sensing device.
With respect to a solid state image sensing device, which can decrease a horizontal transmission rate of electric charge in the HCCD 3, a specific type of solid state image sensing device, which is equipped with two HCCDs and transmits electric charges through a different HCCD 3 in an even number line and an odd number line respectively, is available in a market. However, this type of solid state image sensing device is required to be installed with two HCCDs and to transmit electric charges from the VCCD with distributing the electric charges to two HCCDs. Accordingly, there is existed a further problem such that it is hard to increase pixels more.